1. Field of the Invention
The invention generally relates to wireless data communication; and in particular, the present invention relates to network access protocols used in wireless data communication.
2. Background of the Invention
Network access protocols exist for use in conjunction with a two-way wireless data network such as that shown in FIG. 1 and described in 759340 vldetail in and commonly assigned U.S. patent application Ser. No. 08/542,860, entitled "TWO-WAY WIRELESS DATA NETWORK", of Weijia Wang, now U.S. Pat. No. 5,898,904, issued on Apr. 27, 1999, which is incorporated herein by reference in its entirety.
As shown in FIG. 1, a wireless data network 100 includes a wireless data terminal 101, a cellularized base station 102, message control center 103, interfaces 105-109 to information and communication applications, and radio links 115 and 117. Message control center 103 has a high power transmitter capable of broadcasting through the paging channel to wireless data terminals within the entire service area of wireless data network 100. In this manner, wireless data network 100 is compatible with existing one-way paging services. Message control center 103 also communicates with base station 102 through radio link 117 which is a high power (e.g. 3 watts) transmission.
Base station 102 is one of a number of base stations that are distributed throughout the service area of wireless data network 100, each base station serving a relatively small portion of the service area of wireless data network 100 within its immediate vicinity. The local service area of a base station is sometimes called a cell. The base stations broadcast to wireless data terminals in their respective cells through a local channel. It is desired that the base stations cumulatively serve all locations within the service area of the wireless data network 100. Wireless data terminal 101 communicates with one or more of the base stations, for example base station 102, through link 115. Link 115 needs only provide a low power transmission (e.g. 100 mW) to cover a local area. At a location outside of the local service area of any base station, wireless data terminal 101 is restricted only to receiving messages from the 1-way paging channel. Other details of the operation of the two-way wireless data network can be found in the aforementioned copending patent application entitled "TWO-WAY WIRELESS DATA NETWORK" of Weijia Wang.
Base station 102 in wireless data network 100 uses a channel access protocol which allows all base stations within wireless network 100 to communicate with wireless data terminals over the same packet radio link at the same carrier frequency. An example of such an access protocol is disclosed in and commonly assigned U.S. patent application Ser. No. 08/542,770, entitled "WIRELESS NETWORK ACCESS SCHEME", of Weijia Wang et al., now U.S. Pat. No. 5,721,733, issued on Feb. 24, 1998, which is incorporated herein by reference in its entirety.
The network access protocol described in Weijia Wang et al. provides for high channel utilization rate under both the ALOHA protocol and the carrier sensing multi-access (CSMA) protocol by making use of the FM capture effect. However, the network access scheme in Weijia Wang et al. can be enhanced to increase the bandwidth and channel utilization of the two-way wireless data network described above without costly replacement of existing transceivers.
In these wireless networks, to initiate communication with a wireless data terminal, a wake-up signal is first sent from a message control center (e.g. message control center 103) to the wireless data terminal, before two-way communication is established between a local base station and the wireless data terminal. Because the high power transmitter of the message control center is a shared resource, under high message traffic conditions, an undesirable latency may be incurred. It would be desirable that, under certain conditions, the message control center can be bypassed in initiating local two-way communication, without at the same time surrendering the benefits described above.
Also, in wireless communication, to achieve a higher data rate, a transmitter is typically required to output a higher power. However, to achieve the mobility advantage of a wireless terminal, a wireless data terminal is typically powered by a battery, which must be provided with a reasonable service life-time between recharging operations. Thus, an increased data rate without a corresponding increase in the power requirement of the wireless data terminal is desired.
Other mechanisms for extending the wireless data terminal's limited service life-time between recharging are also desired.